Data storage disks are produced using a disk replication process. A master disk is made having a desired surface relief pattern formed therein. The surface relief pattern is typically created using an exposure step (e.g., by laser recording) and a subsequent development step. The master is used to make a stamper, which in turn is used to stamp out replicas in the form of replica disk substrates as part of a disk molding process. As such, the surface relief pattern, information and precision of a single master can be transferred into many inexpensive replica disk substrates.
Conventional mold assemblies typically include a fixed side and a moving side. The stamper is typically attached to either or both sides of the mold assembly for replicating a desired surface relief pattern (i.e., lands, grooves and/or pits) into the replica disk substrate. A movable gate cut may be provided for cutting a central opening in the replica disk substrates. The stamper may be secured to the moving side using an inner holder and outer holder, wherein the inner holder and outer holder fit over the stamper. Several more tooling parts may be located at the center of the mold assembly to assist in ejection of the component.
During the disk molding process, a resin, typically optical grade polycarbonate, is forced in through a sprue channel into a substrate cavity within the mold assembly to form the replica disk substrate. The surface relief pattern or formatted surface is replicated in the replica disk substrate by the stamper as the cavity is filled. After filling, the gate cut is brought forward to cut a center hole in the replica disk substrate. After the replica disk has sufficiently cooled, the mold assembly is opened and the gate cut and a product eject may be brought forward for ejecting the formatted replica disk substrate off of the stamper. The inner holder and outer holder may be removable to allow changeout of the stamper.
In injection—compression molding, while the resin is forced into the substrate cavity of the mold assembly by the molding press, injection pressure overcomes clamp force causing mold to open a small amount (commonly termed “mold blow”). Pressure is then increased to the mold assembly to clamp the mold shut, forcing the resin into the microscopic surface relief pattern of the stamper (which contains the reverse image of the desired replica disk surface relief pattern). Thus, the above process is commonly termed “injection compression” or “micro-coining”.
For disk formats utilized in flying head applications, as disk capacity increases the design tolerances for the desired surface relief pattern become more critical. For high capacity disks the flying heads may be required to pass closer to the disk substrate, requiring tighter disk specifications, including a reduction or elimination of disk surface geometry imperfections.
One such disk surface geometry imperfection is the thickness increase that has been consistently seen at the outer edge of a typical polymeric optical disk substrate. This phenomenon has been given the name “edge wedge” or “ski jump” effect. This “edge wedge” is shown schematically in prior art FIG. 1 and FIG. 2. “Edge wedge” causes problems in a hard disk-type system where a read/write head is designed to fly as close as possible (i.e., on the order of 1–5 micro-inches) to the surface of the media substrate. For example, one typical polycarbonate disk substrate has an average thickness of about 2 mm (as shown at T1), and a radius of 65 mm. The “edge wedge” effect is primarily seen at the outer radius region of the polycarbonate disk between 62 mm and 65 mm, where the maximum substrate thickness (i.e., bump height) T2 at radius 65 mm is approximately 10–20 microns thicker than the substrate thickness at radius 63 mm. When the bump height differential (T2−T1) divided by the average thickness (T1) exceeds 0.01 (1 percent), read/write flyability problems are often encountered.
The “edge wedge” phenomenon can be attributed to many factors. During cooling of the disk substrate in the mold, the plastic “freezes” at different rates in different radii of the part. The outer edge of the disk substrate freezes through the thickness faster due to its contact with the cold outer holder. Other factors include the tendency of the disk substrate material molecules to be in substantial radial alignment near the center of the disk substrate, and relatively misaligned near the outer edge due to the mold filling process. All of these factors result in the outer edge of the disk substrate exhibiting a greater thickness than the remainder of the disk substrate.
The “edge wedge” phenomenon can be further described as follows. When the optical disk substrate is molded in the micro-coining process described earlier, the densification that is associated with the cooling plastic is accommodated through a corresponding reduction in the mold cavity size (as opposed to reduction in mold cavity pressure as in conventional injection molding). During the filling phase, the mold halves are forced slightly apart by the fluid pressure applied from the injection unit. As the plastic in the mold cools, it shrinks, and the mold halves translate into closer proximity as the press maintains a constant clamp force or pressure on the solidifying melt. The part will freeze through the thickness at slightly different rates at different radii. Regions that are frozen fully through early will do so while the mold is blown to a greater extent or whilst the cavity z-dimension is larger in the earlier phases of the micro-coining molding process. These fully frozen regions will then strain due to clamp force in an elastic fashion (meaning that the solid material will spring back upon release of applied force). Regions that remain liquid at the center will strain in a viscous fashion (this is non-recoverable strain) and will continue to shrink in size or density as they more slowly solidify and eventually take on a thickness of a smaller cavity dimension from later in the coining/cooling process. Therefore, after the clamping force is removed, the early-freezing regions (outer circumference areas) will spring back to a larger thickness than those areas that froze completely through later in the process (the inner disk area).
For a traditional optical disk, where information is stored “substrate incident” the “edge wedge” effect does not present a major problem. In substrate incident applications, a transparent protective layer covers the information layer of the disk. An optical disk player including a laser light source positioned away from the disk surface, focuses a laser beam through the protective layer at the information layer to access (i.e., read) the data stored on the disk. However, for “flying head” applications where information is stored on a disk surface (i.e., where information is stored “air incident”), a read/write head is flying 1–2 micro-inches above the substrate surface. The “edge wedge” phenomena is associated with a loss of flyability of the read/write head where the outer edge of the head comes into contact with the rising surface of the media substrate, resulting in a “head crash” if the head were allowed to fly over the outer portion of the disk. The outer edge of the disk is unusable for data storage, since the curvature of the surface becomes too great to provide a functional air bearing between the head and the surface of the disk. This limits the capacity, functionality and robustness of the disk data storage system.
Unfortunately, the outer circumference of the disk substrate where the “edge wedge” effect occurs is also the most desirable area for data storage. This outer circumference provides a large area for data storage since the data tracks are larger. Therefore, the need exists to eliminate the “edge wedge” to prevent disk crashes and to increase the useable area of the disk.